MORPHOLOGY AND TAXONOMY OF THE SARCODINA 443 



Contractile vacuoles are present in fresh water forms but are 

 generally absent in marine types. They never have the complex 

 canal system such as found in some flagellates and ciliates and are 

 rarely fixed in position. Gas vacuoles are present in some of the 

 testate fresh water forms (Arcella). 



The majority of Rhizopoda are multinucleate both in fresh water 

 and marine species, the multiple number due mainly to repeated 

 nuclear division aided, in Mycetozoa, by plasmodium formation 

 through fusion. The structure of nuclei is too varied for a general 

 description but the vesicular, endosome tvpe predominates (see 

 p. 50). 



Nutrition is holozoic and some progress has been made in working 

 out processes of digestion, digestive ferments, etc. (see Chapter V). 

 Living organisms are captured by pseudopodia or entrapped in the 

 protoplasmic network where they are digested. Cyclosis is invar- 

 iable and the various protoplasmic granules, digested food sub- 

 stances, etc., are thoroughly mixed. 



Reproduction occurs in a variety of ways by division which may 

 be either equal or binary division, budding division, unequal division 

 or budding, and multiple division or sporulation. So-called budding 

 division is the most characteristic and is a form of division appar- 

 ently limited to the Rhizopoda (see p. 225). 



Sexual processes are well developed, microgametes being formed 

 in the majority of cases, which will be reviewed in connection with 

 the several classes. 



The classification adopted is an extension of that used by Minchin 

 and includes as primitive forms those questionable Heliozoa-like 

 types which many authors (e.g., Reichenow-Doflein) include with 

 the Ileliozoa. 



Sub-class I. PROTEOMYXA Laxkester. 



There are but few common characteristics in this group of primi- 

 tive forms; the most widely spread feature apparently is the usual 

 occurrence of ray-like pseudopodia which recall the appearance of 

 Ileliozoa. These have no axial filaments however, and frequently 

 branch or partially anastomose. Flagellated swarm-spore stages are 

 common but the life history is known in few cases. An approach 

 to the Mycetozoa is seen in forms like Labyrinthula where the small 

 spindle-shape cells bear long filose pseudopodia which fuse to form 

 a net-like mesh. Most of them are parasites on lower algae and 

 Protozoa. 



Family 1. Labyrinthulidae Ilaeekel. This family is composed of 

 different species of the genus Labyrinthula which may lie intracellu- 

 lar parasites in diatoms, Vaucheria, Spirogyra, etc. They frequently 

 become associated in groups or pseudoplasmodia and reproduce by 



